1. Field of the Invention
The present invention concerns a flextensor transducer. It can be applied to the emission or reception of acoustic waves in liquids.
2. Description of the Prior Art
Known flextensor transducers are piezoelectrical transducers generally consisting of a flexible shell that is impervious, with a cylindrical side wall having an elliptical cross section, put into vibration by one or more pillars or bars of piezoelectrical cells made of ceramic. Each pillar is held compressed between those opposite parts that are furthest away from the lateral wall. In emission, an ac electrical field is applied in the longitudinal direction of each pillar and the resultant motion, which takes place along the longitudinal axis of each pillar, is retransmitted, in amplified form, to the surrounding liquid medium. The amplitude of this motion is at its maximum in the plane generated by the small axes of the ellipses formed by each cross section.
The compression of the piezoelectric cells of each pillar is necessary to prevent the breakage of the ceramic when the pillars are subjected to stretching forces.
According to a first known embodiment, this prestressing is given directly by the shell during the assembly of the pillars. Before assembly, housings designed in the shell for the pillars have smaller lengths than those of the pillars. To position the pillars, it suffices to apply two opposite external forces to those facing parts that are closest to the side wall to compress the shell at this place and, through the elastic deformation of this shell, to cause an increase in the length of the housings that is exactly sufficient to enable the installation of the pillars. The prestressing force is applied when the action of the two external forces is eliminated. The pillars then remain compressed in their housing between the parts of the internal side wall of the shell in contact with their ends.
To obtain accurate functioning of the transducers at a determined depth, this embodiment requires that the amplitude of the two external forces should be given a value greater than that normally exerted by the hydrostatic pressure at this depth. This has the drawback of restricting the use of these types of transducers to the depths for which the prestressing force of the pillar can still be ensured, to prevent the breakage of the ceramic forming the piezoelectric cells.
According to a second known embodiment, the prestressing force of each pillar may be obtained by means of a rod going through each pillar along its longitudinal axis, the ends of the rod being held by being bolted to the shell. However, in this case, the hydrostatic pressure exerts a tensile stress on each pillar which, when it is excessive, causes breakage of the ceramic forming the piezoelectric cells.
Finally, according to a third known embodiment, a description of which may be found in the U.S. Pat. No. 4 420 826, the piezoelectric cells may be stacked along a prestressing rod which is not fixed by its ends to the rod. The stack is held by two rails so as not to be subjected, as in the previously described embodiment, to a tensile stress directed along the longitudinal axis of the pillar. However, here again, when the submersion of the transducer is such that one or two sides of the pillar are no longer in contact with the shell, the transducer can no longer work properly.